I used a black contour marker for glass and porcelain. The tip is 1-2 mm thick and the color is very opaque. The hard part is to be steady at hand, but like I mentioned earlier, I actually prefer when it looks handmade. Let your decorated porcelain dry for 4 hours and then be cured in the kitchen oven at 160°C for 90 minutes, to be dishwasher safe. Do you remember the bowls I painted last winter? They still look as nice! :)
There is nothing more fun than being able to customize your own mugs with your own designs, sayings and more! Don't go to the expense of having them printed. Just learn how to make a custom mug with a sharpie.